Die-casting apparatus with ceramic shot duct liner

ABSTRACT

Die-casting apparatus, primarily for ferrous metals, in which the shot duct comprises a ceramic liner supported by a casing, the liner and the casing being of complementary tapered form and assembled together as a press fit. The casing may be shrunk onto the liner and may be so dimensioned that, at the working temperature, the liner is under radial compression.

United States Patent Carver et al. [451 May 23, 1972 [54] DIE-CASTING APPARATUS WITH [56] References Cited CERAMIC SHOT DUCT LINER UNITED STATES PATENTS [72] Inventors: Bryan George Carver, Birmingham;

1,810,091 6/1931 Sregle ..29/525 X 23" 2,293,089 8/1942 Wainer g 2,609,318 9/1952 Swentzel ..266/43 x [73] Assignee: G.K.N. Group Services Limited, Warley, 3,515,203 6/1970 Parlanti et al ..164/3 1 2 England 3,516,480 6/1970 Woltering 164/312 [22] Filed: 1970 Primary Examiner-Robert D. Baldwin [21] Appl 2 220 Att0rneyMilton J. Wayne [57] ABSTRACT [30] Foreign Application Priority Data I Die-casting apparatus, primarily for ferrous metals, in which Oct. 25, 1969 Great Brltarn ..52,378/69 the Shot duct comprises a ceramic liner Supported by a casing the liner and the casing being of complementary tapered form [52] U.S.Cl ..l64/3l2, 164/303 and assembled together as a press fit The casing may be [51] Int. Cl ..B22d 17/10 shrunk onto the neland may be so dimensioned that at the [58] Field of Search ..164/3l2, 313, 314, 315, 316, working temperature, m liner is under radial compression 9 Claim, 2 Drawing Figures X l lo I DIE-CASTING APPARATUS WITH CERAMIC SHOT DUCT LINER BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an apparatus for casting metal into a cavity formed between separable repeatedly usable dies and in which shots of molten metal are fed into the cavity under pressure, as distinct from merely under gravity, along a duct (herein called the shot duct) by a piston movable therein, said shot duct communicating at one end (herein called the delivery station) with the die cavity, and being so formed or constructed as to be able to receive at another position (herein called the receiving station) spaced longitudinally of the molten metal to be fed along the shot duct into the die cavity. Such apparatus is hereinafter referred to as die-casting apparatus of the kind specified".

The invention is applicable primarily to die-casting apparatus of the kind specified for casting ferrous metal, that is cast iron or steel, but may be applied generally to apparatus for casting other metals, more especially those in relation to which similar or analogous problems to those hereinafter described arise.

2. Description of the Prior Art.

Ferrous metals require to be cast at a high temperature which typically is in the region of 1300 C for cast iron and in the region of 1600 C for steel.

The conveying of a ferrous metal at these temperatures along the shot duct has been found to produce rapid deterioration of the shot duct leading to a shortened service life.

In particular various forms of erosion or failure have been observed at different positions along the shot duct. Thus the interior surface of the shot duct tends to be eroded by penetration into the surface layer of pieces of solid state skin formed on the molten metal immediately adjacent to the inner surface of the shot duct, due possibly to the breaking up of this skin during the forcing of the shot of molten metal by movement of the piston along the shot duct to force the metal under pressure into the die cavity. This phenomenon has been observed to take place predominantly adjacent to the delivery end of the shot duct.

The action of the solidified skin on the surface layers of the material of which the shot duct is composed is partly analogous to abrasion by a solid state abrasive substance, and partly analogous to the establishment of elementary friction welds particularly if the material utilized for the shot duct is a metal having welding compatability with the metal undergoing casting.

In contrast with this it has been observed that the interior surface of the shot duct at the receiving station is consistent with physical and/or chemical erosion by impact and contact with the molten metal to be cast when the latter is first delivered into the shot duct.

For example, there is marked erosion in a region of impact of the molten metal at the receiving station on the interior surface of the shot duct (such metal being delivered thereinto through an opening formed conveniently in the upper wall of the shot duct) when the cast metal is cast iron and the shot duct is formed of steel. It is believed that this erosion is due to the greater percentage of carbon present in the cast iron than in the steel, the iron constituent of the latter tending to become transferred tothe cast iron to bring about a greater degree of equality in the carbon concentration of the two metals.

Erosion at the region of impact is also believed due to fluid friction between the molten metal to be cast and the solid state material of which the shot duct is composed.

An intennediate section of the shot duct lying between the delivery and receiving stations is also subjected to deterioration which tends to take place by way of fracture, and observations which we have carried out are consistent with such fracture having been brought about by thermal fatigue.

SUMMARY OF THE INVENTION The present invention is based on the concept of providing a component to present the interior surface of the shot duct and formed of a material which is intrinsicallv capable of withstanding these conditions but which, in itself, cannot be produced, or economically produced, in a form which will provide the required transverse rupture strength.

According to the present invention, we provide a shot duct which comprises a liner made of a material or materials selected to withstand contact with the metal to be cast in respect of erosion phenomena and thermal fatigue without necessarily having the requisite transverse rupture strength, and a casing supporting the liner externally and made of a material and to dimensions providing alone, or in combination with the liner, the required transverse rupture strength, the liner affording an external surface of slightly tapered form and the casing affording an internal surface having a taper complementary to that of the liner so that the liner and the casing are assembled together as a press fit and the liner is supported by the casing at all positions along its length.

Thus, in one form of the invention, the liner may be made of a ceramic material which is inert with respect to the metal to be cast and which presents the requisite surface hardness to withstand erosion by abrasive action of the metal to be cast in moving along the shot duct.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described, by way of example, with reference to the accompanying drawings wherein:

FIG. 1 shows diagrammatically an injection moulding machine in side elevation, and

FIG. 2 shows a vertical cross-section through the axis of the shot duct thereof on a larger scale.

DESCRIPTION OF THE PREFERRED EMBODIMENT As seen in FIG. 2, the apparatus comprises separable dies 10 and 11 which define a die cavity 12 and which separate along a parting plane X. Such dies can be moved between a closed position, as shown in FIG. 2, and an open position in which the die 1.1 moves to the left away from the die 10 by any suitable mechanism, either mechanical or hydraulic of known construction.

FIG. I shows a known form of machine employing a mechanical mechanism for closure of the dies 10 and II. This machine includes a bed 1 having a mounting member 2 which carries the die 10 in a fixed position. The movable die 11 is carried on a slide plate 3 which is slidable along horizontally extending bars 4 which are fixed in position relative to the bed 1. The slide plate 3 is movable along the bars 5 bv means of a toggle mechanism indicated generally at 5. The toggle mechanism 5 is operable by means of a piston and cylinder unit 6 which is carried by a longitudinally adjustable mounting member 7. A connecting rod 6a extends from the unit 6 to the toggle mechanism 5. Also fixedly mounted on the bed 1 there is a piston and cylinder unit 8 having a connecting rod 8a for operation of the shot duct 14 which is hereinafter described in detail.

The dies 10 and 11 are made of a material which is capable of repeated use to form cast metal articles in the die cavity 12.

When the metal to be cast is a ferrous metal, such as cast iron or steel, the dies 10 and 11 are either made of, or lined with, a metal of a higher melting point or have a rate of heat dissipation, either natural or produced by cooling means provided in association therewith, which maintain them at a sufficiently low temperature to enable them to withstand repeated use.

Preparatory to casting metal into the die cavity 12, a medium may be sprayed onto the surface of the die cavity to facilitate stripping the cast article therefrom and a suitable medium for this purpose is graphite suspended in a suitable carrier, or carbon black deposited from an acetylene torch.

One or both of the dies is formed with a channel or gate 13 by means of which the metal to be cast is fed into the die cavity upwardly, preferably into the lowermost part of the die cavity.

The metal is fed to the lower end of the gate 13 by means of the shot duct 14, and which is formed or constructed to define a horizontal, or approximately horizontal, bore 15 conveniently of circular shape in cross-section and of uniform diameter throughout its length. In some cases the bore 15 could be inclined to the horizontal.

The cast metal is delivered into the bore 15 at a receiving station 16 and is moved along the bore by a piston 17 connected to a suitable operating mechanism, either mechanical or hydraulic, such as the piston and cylinder unit 8, the metal then being forced from the bore 15 at a delivery station 18 to pass upwardly through the gate 13 under pressure.

The pressures employed may vary widely but it is contemplated that pressures of as much as 5,000 to 6,000 lbs. per square inch may be obtained during the operation of feeding the molten metal into the die cavity.

Again, delivery of molten metal into the bore may be effected in a variety of ways, one of which is illustrated diagrammatically and consists in pouring the metal from a pot 19 through an opening 20 leading to the receiving station 16, the metal being maintained at the required temperature, for example in the region of 1,300 C for cast iron and l,600 C for steel, by any suitable heating means provided in association with the pot.

Referring now specifically to the construction of the shot duct 14, this comprises a liner 2] which extends for the whole of the shot duct and is formed of a material selected to provide the requisite resistance to deterioration by contact with the metal to be cast and by movement of thismetal by means of the piston 17 along the bore 15.

Ceramic and cermet materials are especially suitable for this purpose and it is believed that the material silicon nitride is particularly suitable. Possible alternatives are silicon carbide and zirconia and other comparable refractory materials.

The liner may be formed as a casting of the ceramic material, although, if necessary, the dimensions of the interior and/or the exterior surfaces may be achieved to the required degree of accuracy by grinding.

The inner surface is of constant diameter and the outer surface is tapered having its smallest diameter at the forward or left-hand end as illustrated and typically a taper of 0.002 to 0.003 inch on the diameter per inch length. The direction of the taper however could be reversed.

The liner is supported externally by a casing which as shown is formed of a plurality of components, although a single unitary casing could be provided if desired.

As illustrated, the innermost component 22 of the outer casing is of generally cylindrical form and extends from end to end of the liner 21. The internal surface of the component 22 is formed with a taper complementary to that of the external surface of the liner and the two components are assembled one within the other as a press fit, so that, due to the taper, the liner is supported at all positions along its length and may, if desired, designedly be subjected to a degree of prestress.

The material employed for the innermost casing component 22 may be a metal, for-example steel. This may be a mild steel or a steel composition normally employed for the construction of hot working dies, a typical composition being according to the A.l.S.I. specification H13, namely carbon 0.03 0.4 percent; manganese 0.02 0.4 percent; silicon 0.08 1.2 percent; chromium 4.7 5.5 percent; vanadium 0.8 1.2 percent and molybdenum 1.2 1.75 percent; balance iron. Alternatively other refractory metals may be employed such as the so called superalloys. It may also be possible to employ graphite having an oxidation-preventing coating of, for example, silicon carbides.

The thermal coefficient of line expansion of this steel is considerably in excess of, that afforded by the ceramic materials mentioned and accordingly it is preferred that the temperature of the component 22 be raised to a suitable value preparatory to assembling the liner 21 therein, so that there is shrinkage of the component 22 onto the liner, preferably to an extent such that at the working temperature there is still some degree of inward radial compression exerted on the liner by the component 22 of the casing.

As illustrated, the casing further comprises a forward outer component 23 having a flange 24 by means of which it can be secured by screws 25 or other fastening elements to the die 10. The casing further comprises a rear outer component 26, the forward portion of which is concentric with the component 23 and the rearward portion of which embraces the innermost component 22 of the casing throughout the remainder of the length of the latter, and includes an inwardly directed flange 26a engaging the end faces of the liner 21 and inner casing component 22. The outer components of the casing may be secured together in any suitable manner as, for example, by screws 29 or other fastening elements extending through the flange 30 into the rear end of the forward outer component 23.

The main function of the casing is to support the liner 21 and thereby to provide the requisite transverse rupture strength for the shot duct as a whole not provided by the liner 21 alone.

Further, it will be noted that due to the concentric assembly of the inner and outer components of the casing, it is possible readily to control the rate of heat transmission from the interior surface of the liner 21 in an outward direction, and also to control the magnitude of the heat sink presented at any position along the length of the shot duct by the components of the outer casing.

The piston 17 may also be formed of a ceramic material, for example the same material as that utilized for the liner 21. The piston 17 also serves as a means for ejecting a plug 28 of solidified metal which remains in the end portion of the bore 15 adjacent to the delivery station, but which is free to be ejected upon movement of the die 11 in a direction to the left away from the die 10.

Although the liner 21 has been illustrated as being formed in one piece, it can be formed as a plurality of longitudinally successive structurally separate components which may all be made of the same material or which may be made of differing materials selected to suit the different operating conditions which arise at different positions along the length of the shot duct. Thus, adjacent to the delivery station, the shot duct liner may be formed of a material which is selected to present a hard surface at the operating temperature resistant to penetration by fragmentation and displacement of a solid state skin formed in the metal undergoing casting adjacent to the interior surface of the shot duct at this station. On the other hand, adjacent to the receiving station, the shot duct liner may be formed from a material which does not enter into chemical and/or physical reaction with the metal undergoing casting. Preferably also the shot duct includes an intermediate component situated between those at the delivery and receiving stations, such intermediate components being formed from a material selected to have good resistance to thermal fatigue.

' We claim:

1. In apparatus for casting metal comprising a. separable repeatedly usable dies defining in combination a cavity,

b. means for moving said dies relatively towards and away from one another between open and closed positions,

c. a shot duct having at one end a receiving station for receiving shots of molten metal and at its other end a delivery station communicating with said cavity, and

d. a piston movable in said shot duct to feed said shots of molten metal into said cavity, the improvement wherein e. said shot duct comprises a liner made of ceramic material selected to withstand contact with the metal to be cast in respect of erosion phenomena and thermal fatigue without having the requisite transverse rupture strength, and a casing supporting the liner externally and made of a material and to dimensions providing in combination with the liner the required transverse rupture strength, and

the liner affording an external surface of slightly tapered form and the casing affording an internal surface having a taper complementary to that of the liner whereby the liner and the casing are assembled together as a press fit so that the liner is supported by the casing at all positions along its length.

; 2. Apparatus as claimed in claim 1 wherein said casing is a metal member shrunk onto the liner by being initially asseml'iled therewith while raised to a high temperature wherebv at the working temperature of the shot duct, the line is under radial compression by the casing.

3. Apparatus as claimed in claim 1 wherein said piston is formed of ceramic material.

4. Apparatus as claimed in claim 1 wherein the taper afforded by the external surface of the liner is between 0.002 and 0.003 inch per inch length.

l i i 5. Apparatus as claimed in claim 1 wherein the ceramic material of said liner is selected from the group comprising silicon nitride, silicon carbide, and zirconia.

6. Apparatus as claimed in claim 1 wherein the ceramic material of said liner is a cermet material.

7. Apparatus as claimed in claim 1 wherein said liner has a radiused comer at the junction of the internal surface thereof and the end thereof at said delivery station.

8. Apparatus as claimed in claim 1 wherein said liner extends into an aperture in one of said dies so as to be partly supported thereby.

9. Apparatus as claimed in claim 1 wherein said casing comprises an inner casing component extending over and in contact with the whole length of said liner and at least one outer casing component extending over and in contact with said inner casing component at least externally of the die. 

1. In apparatus for casting metal comprising a. separable repeatedly usable dies defining in combination a cavity, b. means for moving said dies relatively towards and away from one another between open and closed positions, c. a shot duct having at one end a receiving station for receiving shots of molten metal and at its other end a delivery station communicating with said cavity, and d. a piston movable in said shot duct to feed said shots of molten metal into said cavity, the improvement wherein e. said shot duct comprises a liner made of ceramic material selected to withstand contact with the metal to be cast in respect of erosion phenomena and thermal fatigue without having the requisite transverse rupture strength, and a casing supporting the liner externally and made of a material and to dimensions providing in combination with the liner the required transverse rupture strength, and f. the liner affording an external surface of slightly tapered form and the casing affording an internal surface having a taper complementary to that of the liner whereby the liner and the casing are assembled together as a press fit so that the liner is supported by the casing at all positions along its length.
 2. Apparatus as claimed in claim 1 wherein said casing is a metal member shrunk onto the liner by being initially assembled therewith while raised to a high temperature whereby at the working temperature of the shot duct, the line is under radial compression by the casing.
 3. Apparatus as claimed in claim 1 wherein said piston is formed of ceramic material.
 4. Apparatus as claimed in claim 1 wherein the taper afforded by the external surface of the liner is between 0.002 and 0.003 inch per inch length.
 5. Apparatus as claimed in claim 1 wherein the ceramic material of said liner is selected from the group comprising silicon nitride, silicon carbide, and zirconia.
 6. Apparatus as claimed in claim 1 wherein the ceramic material of said liner is a cermet material.
 7. Apparatus as claimed in claim 1 wherein said liner has a radiused corner at the junction of the internal surface thereof and the end thereof at said delivery station.
 8. Apparatus as claimed in claim 1 wherein said liner extends into an aperture in one of said dies so as to be partly supported thereby.
 9. Apparatus as claimed in claim 1 wherein said casing comprises an inner casing component extending over and in contact with the whole length of said liner and at least one outer casing component extending over and in contact with said inner casing component at least externally of the die. 